Conventionally, a small loss (cavity) formed due to the decay or the like of a tooth has been filled with a metal material. However, recently, because the color tone comparable to a natural tooth color can be provided and an operation is easy, a filling/restoring material containing an acrylic polymerizable monomer, an inorganic filler, and a photopolymerization initiator has been used preferably.
However, the above-mentioned resin-based filling/restoring material has no adhesiveness with respect to a tooth. Therefore, when a cavity of a tooth is filled with only a resin-based filling/restoring material, a gap may be formed between the filling/restoring material and the tooth. This increases a risk that a bacterium enters through the gap or the filling/restoring material comes off the tooth.
Then, the idea of providing a layer of a pretreatment material containing, as a main component, a monomer or the like having an acidic group between the resin-based filling/restoring material and the tooth has been realized. The pretreatment material functions as a surface treatment agent for an enamel and a dentin of a tooth and enhances the adhesion between the filling/restoring material and the tooth. The filling/restoring material and the tooth can be attached to each other without any gap by interposing such a pretreatment material between the tooth and the resin-based filling/restoring material.
It is preferred that the filling/restoring material can be deformed easily when provided with the shape of a tooth and be cured immediately after the completion of filling. Therefore, the filling-restoring material and the pretreatment material contain a photopolymerization initiator, and are cured preferably using visible light that is harmless to a living body.
The operation of restoring a tooth is performed in the following procedure. First, a decayed tooth portion is drilled to form a cavity. Then, the cavity is coated with a pretreatment material. Then, in order to cure the pretreatment material, the portion coated with the pretreatment material is irradiated with visible light. Then, on the layer of the pretreatment material a filling/restoring material is filled. Finally, in order to cure the filling/restoring material, the filling/restoring material is irradiated with visible light.
In the case of the above-mentioned restoration method, it is necessary to irradiate the filling/restoring material with visible light twice. However, it is desired to reduce the number of irradiation with visible light in terms of the alleviation of a burden on a patient and the simplification of a restoration operation.
However, when the pretreatment material and the filling/restoring material are cured concurrently by one irradiation with visible light, a large stress is generated at an adhesion interface due to a large polymerization contraction amount of the filling/restoring material. As a result, there may easily arise a problem that the adhesive strength between the pretreatment material and the filling/restoring material becomes very small or adhesion durability during a long period of time becomes poor.
In order to solve the above-mentioned problem, a dental filling kit has been proposed, which contains a monomer having an acidic group, a monomer having no acidic group, and a polymerization initiator in both the filling/restoring material and the pretreatment material (see, for example, Patent Document 1). In the kit, the filling/restoring material is formed of a composition similar to that of the pretreatment, and hence, the affinity of the interface between the pretreatment material and the filling/restoring material is enhanced, which makes it difficult for the pretreatment material and the filling/restoring material to peel off at the interface. Thus, even in the case where the pretreatment and the filling/restoring material are polymerized concurrently by only one irradiation with light, relatively high adhesive strength is obtained.    Patent Document 1: JP 2006-131612 A (claim 1 or the like)